Re: [PATCH V4 1/2] rust: Add initial cpumask abstractions

[Burak Emir]

On Fri, Apr 11, 2025 at 9:09 AM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
>
> On 02-04-25, 18:00, Burak Emir wrote:
> > On Wed, Apr 2, 2025 at 5:47 PM Yury Norov <yury.norov@xxxxxxxxx> wrote:
> > > On Wed, Apr 02, 2025 at 11:08:42AM +0530, Viresh Kumar wrote:
> > > > +    pub fn set(&mut self, cpu: u32) {
> > > > +        // SAFETY: By the type invariant, `self.as_raw` is a valid argument to `cpumask_set_cpus`.
> > > > +        unsafe { bindings::cpumask_set_cpu(cpu, self.as_raw()) };
> > > > +    }
> > >
> > > Alright, this is an atomic operation. For bitmaps in rust, Burak and
> > > Alice decided to switch naming, so 'set()' in C becomes 'set_atomic()'
> > > in rust, and correspondingly, '__set()' becomes 'set()'.
> > >
> > > I think it's maybe OK to switch naming for a different language. But
> > > guys, can you please be consistent once you made a decision?
> > >
> > > Burak, Alice, please comment.
> >
> > I really like the explicit naming convention that includes "atomic" if
> > an operation is atomic.
> > It seems also consistent with std library.
> >
> > > Regardless, without looking at the end code I can't judge if you need
> > > atomic or non-atomic ops. Can you link the project that actually uses
> > > this API? Better if you just prepend that series with this 2 patches
> > > and move them together.
> >
> > The type &mut self gives it away: the Rust type system enforces
> > exclusive access here due to aliasing rules.
> > So a non-atomic operation is sufficient here.
>
> I should leave it as is then, right ? Don't rename to set_atomic() ?
>

The issue is that your code calls `bindings::cpumask_set_cpu` calls C
`cpumask_set_cpu` which calls `set_bit` which is atomic.
So if it was only about naming, you should call it `set_atomic`.

More important than the name is the meaning: you don't actually *need*
or want to call to call (atomic) `set_bit` underneath, since you have
`&mut self`.
So you should rather introduce and call `bindings::__cpumask_set_cpu`
which would call the C `__cpumask_set_cpu` which calls the
(non-atomic) `__set_bit`.

Example where we need bindings helpers for both atomic and non-atomic
versions: https://lore.kernel.org/rust-for-linux/20250327161617.117748-3-bqe@xxxxxxxxxx/

cheers,
Burak